1. Field of the Invention
The present invention relates to a printing device for printing on an elongated web-like print paper. More particularly, the invention relates to a paper drift correction device for correcting the position of a print paper drifted in the widthwise direction during travel of the print paper within the printing device.
2. Description of the Related Art
In a printing device of the type in which an elongated web-like print paper travels along a predetermined meandering path to reach a printing position, the print paper is liable to drift or shift in the widthwise direction. To correct the widthwise drift of the print paper, conventional printing devices employ a correction device having a sensor that detects the side edge of the print paper. With the correction device, the print paper is moved back to the right position when the output from the sensor indicates that the side edge of the print paper is drifted from a reference position.
However, the cut condition in the side edges of the print paper differs in different manufacturers and in different slots of paper produced even by the same manufacturer and also differs depending upon a side edge cutting machine used. For the print papers with side edges that are not cut to the same condition, the sensor outputs a signal to correct the widthwise position of the print paper. Moving the print paper in accordance with the output of the sensor may result in incorrect positioning of the print paper.
In view of the foregoing, it is an object of the invention to provide a printing device that is capable of stably transporting an elongated web-like print paper while not allowing the print paper to drift in the widthwise direction.
To achieve the above and other objects, there is provided an improved printing device for printing on an elongated web-like print paper. The print paper has a width and side edges substantially perpendicular to a widthwise direction of the print paper. A printing device includes a paper conveying mechanism for conveying the print paper along a predetermined path, the print paper being conveyed so that side edges are in coincidence with a paper conveying direction; a first sensor disposed at a first position in the predetermined path, for sensing a position of a side edge of the print paper and outputting a first signal; a second sensor disposed at a second position in the predetermined path, the second position being apart a predetermined distance from the first position and downstream of the first position with respect to the paper conveying direction, the second sensor sensing a position of the side edge of the print paper and outputting a second signal; delaying means for delaying outputting the first signal by a predetermined period of time corresponding to a paper conveying time for conveying the print paper from the first position to the second position; computing means for computing a difference between the first signal output from the delaying means and the second signal and outputting a difference signal; a widthwise moving mechanism for moving the print paper in the widthwise direction; and a controller for generating a drive signal based on the difference signal and outputting the drive signal to the widthwise moving mechanism so that the widthwise moving mechanism is driven in response to the drive signal and corrects the position of the print paper in the widthwise direction.
Averaging means may further be provided for computing an average of difference signals output from the computing means over a predetermined period of time. In this case, the controller generates the drive signal based on the averaged difference signal.
A low-pass filter may be connected to the output of the computing means for removing a high frequency component contained in the drive signal. In this case, the controller generates the drive signal based on the filtered drive signal.